So, my '85 D150, with the 318 5.2L motor dies once it has warmed up. I assumed it was the O2 sensor, so I went looking in the exhaust manifold. No O2 sensor?? There are only three intrusions into the manifold or pipes. The first two are ports for the Air Injection System. The third is on the passenger side, combined with the Air system and is a what I assume is a butterfly valve attached to an armature with a connection to a round ?? which has a vacuum line running into it. The vacuum line runs up to one of two? Coolant Vacuum Switch Cold Closed valves. So, three questions: What's the round thing, because the armature is broken therefore it's not working. Second, why a isn't there an O2 sensor? Finally, why else might the truck die once warm? Any help is greatly appreciated.